Can a Martial Adept Superiority die be used on a melee/ranged spell attack?

Question

Say, you have a Hexblade Warlock, or a War Magic Wizard - or a melee/combat focused bard, any caster will do - and you take the feat Martial Adept, granting you a superiority die. If you use a spell - Eldritch Blast, for example - can you then apply a Superiority die to turn it into a tripping attack?

Answer 1

Each maneuver indicates what triggers it. Trip Attack requires a weapon attack.

Note the first sentence of Trip Attack: "when you hit a creature with a weapon attack..." (PHB 74, emphasis mine).

An eldritch blast is not a weapon attack, so you may not use the Trip Attack maneuver with it.

Notably, Feinting Attack would apply to one casting eldritch blast, as it augments your next attack roll. And the maneuvers that aren't really keyed off of attacks at all, like Parry or Evasive Footwork, are not precluded by eldritch blast.

Answer 2

Eldritch Blast and other spell attacks do not qualify for use with a Battle Master maneuver that specify "weapon attack".

Every Battle Master maneuver affecting an attack either explicitly state "weapon attack" or do not in their description, which an Eldritch Blast is not.

Example

Precision Attack. When you make a weapon attack roll against a creature, you can expend one superiority die to add it to the roll. You can use this maneuver before or after making the attack roll, but before any effects of the attack are applied.

The following maneuvers function with non-weapon attacks

• Commander's Strike When you take the Attack action on your turn, you can forgo one of your attacks and use a bonus action to direct one of your companions to strike. When you do so, choose a friendly creature who can see or hear you and expend one superiority die. That creature can immediately use its reaction to make one weapon attack, adding the superiority die to the attack's damage roll.
  • Technically a viable option for a spellcaster. Taking the Attack Action doesn't require you to use a weapon attack if you give up your only attack to Commander's Strike.
• Evasive Footwork When you move, you can expend one superiority die, rolling the die and adding the number rolled to your AC until you stop moving.
• Feinting Attack You can expend one superiority die and use a bonus action on your turn to feint, choosing one creature within 5 feet of you as your target. You have advantage on your next attack roll this turn against that creature. If that attack hits, add the superiority die to the attack's damage roll. The advantage is lost if not used on the turn you gain it.
• Parry When another creature damages you with a melee attack, you can use your reaction and expend one superiority die to reduce the damage by the number you roll on your superiority die + your Dexterity modifier.
• Rally On your turn, you can use a bonus action and expend one superiority die to bolster the resolve of one of your companions. When you do so, choose a friendly creature who can see or hear you. That creature gains temporary hit points equal to the superiority die roll + your Charisma modifier.

Answer 3

Eldritch Blast
Make a ranged spell attack. [...]

Eldritch Blast is a ranged spell attack.

Trip Attack
When you hit a creature with a weapon attack, [...]

Trip Attack requires you to make a weapon attack, so spell attack, including Eldritch Blast does not work.

Note that all Battle Maneuver that requires you to make an attack clearly state weapon attack, so you can't use Eldritch Blast or any spell attack in combination with Battle Maneuver, except Feinting Attack.

You can combine it with casting Booming Blade or Green Flame Blade, since it requires you to make a melee weapon attack.